1. Field of the Invention
Exemplary aspects of the present invention generally relate to a light projecting device, an image reading device using the same, and an image forming apparatus using the image reading device.
2. Description of the Background Art
Generally, an image reading device such as a scanner employed in image forming apparatuses, for example, a facsimile machine, a copier, or a multi-functional system having a plurality of these functions, uses a light source to illuminate a document with light. The reflected light from the document is then reflected by a plurality of mirrors and directed onto an image pickup device such as, for example, a CCD image sensor.
In order to facilitate an understanding of the related art and of the novel features of the present invention, one example of a related-art image reading device is illustrated with reference to FIGS. 13 and 14. FIG. 13 is a side schematic view of the related-art image reading device. FIG. 14 is a perspective schematic view of the related-art image reading device.
The related-art image reading device 101 is a generally-known reduced optical system that includes a cylindrical xenon lamp 104 serving as a light source. Light projected by the xenon lamp 104 illuminates a not-shown document on a contact glass 102, and the diffuse reflection light from the document is directed to an image pickup device 110. The image pickup device 110 then converts the received light into electric signals.
The image reading device 101 includes a first carriage 103 and a second carriage 106. The first carriage 103 includes a first mirror 105 that deflects reflected light from the document. The second carriage 106 includes a second mirror 107 that further deflects the light from the first mirror 105, and a third mirror 108. The first carriage 103 and the second carriage 106 travel on a rail in the image reading device 101 to scan the document.
With such an image reading device, there is increasing market demand for a short rise time, low energy consumption, prolonged product life of the light source, and so forth. Thus, in recent years, a light source using a light-emitting element such as a light-emitting diode, also known as an LED, is often employed in the image reading device in place of the xenon lamp 104.
As illustrated in FIG. 15, in the image reading device using the LED, a bracket 121 that is V-shaped in cross-section is attached to the first carriage 103. An LED 122 and a circuit board (hereinafter referred to as an LED board) 123 that drives the LED 122 are mounted on the bracket 121.
A reflecting plate 118 (hereinafter referred to as a reflector) is mounted on the first carriage 103. The reflector 118 reflects light from the xenon lamp 104 to optimize an illumination distribution and eliminate shades when reading a document having an irregular surface, for example, a document having a pasted sheet or the like.
In the image reading device using the reduced optical system, the distance from the document surface to the image pickup device is relatively long, so that attenuation of the light projected from the light source between the document surface and the image pickup device becomes significant. Therefore, when using the LED in the image reading device, the intensity of the LED 122 needs to be increased. For this reason, a plurality of LEDs 122 is disposed in a main scan direction of the document to increase the light intensity.
In a case in which the LEDs 122 are arranged in an array, the LEDs 122 are obliquely arranged facing the document surface by the bracket 121. It is desirable that the illumination distribution on the document surface in a sub-scan direction be within an irradiation region E shown in FIG. 16. In other words, it is desirable that the light illuminate only an actual read region of the document. FIG. 16 shows the illumination distribution of the related-art image reading device using the LEDs.
However, even if the LEDs 122 are obliquely arranged and the light projected from the LEDs is reflected by the reflector 118 as described above, the light illuminates areas other than the irradiation region E. In such a case, when reading an image including a solid black portion between white portions, for example, the light reflected by the white portions in the irradiation region E enters the image pickup device 110, causing an output value of the solid black portion to increase and thus preventing accurate reproduction of the solid black portion.
To counteract such a difficulty, some related-art image reading devices include a light projecting device including a light guiding member disposed on the radiation surface of the LED array in a main scan direction to guide the light projected from the LEDs to the irradiation region in order to even out the illumination distribution in the irradiation region. With this configuration, the light is guided using a total reflection from both top and bottom surfaces of the light guiding member. Similarly, the amount of peripheral light at the end portions of a document, which typically decreases, can be increased by using the total reflection of the end surface (side surface) of the light guiding member in the main scan direction.
As noted the amount of light decreases from the end portion of the document. Thus, the amount of light available at the end portion is less than that of the center of the document. By contrast, when using the total reflection of the end surface (the side surface) of the light guiding member, a uniform amount of light can be obtained. In this case, it is desirable to dispose the farthest-end LED near the end surface (the side surface) of the light guiding member.
In order to securely hold the LED board and the light guiding member, one related-art approach proposes to position and fix the LED board and the light guiding member using screws. Screw holes are formed on the LED board and the light guiding member. However, the related-art approach suffers from a drawback in that because the screw holes are formed near the end surface of the light guiding member, the total reflection of the light from the LED at the side surface of the light guiding member (the end surface in the main scan direction) is hindered, thereby posing a risk of light loss. Furthermore, forming the screw holes near the end surface of the light guiding member easily causes welding trouble such as weld lines during the molding process, thereby increasing the time required for the molding process and reducing the yield. This ultimately causes an undesirable cost increase.
Furthermore, in order to form the screw holes, the length of the light guiding member is extended. As a result, the distance between the LED at the farthest end and the end surface of the light guiding member increases, thereby complicating efforts to use the total reflection at the end surface. In order to achieve an equal amount of light in the main scan direction, it is necessary to increase either the number of the LEDs or the length of the LED array.
Another related-art approach proposes using a mounting table to hold the LED array and the light guiding member, and a cover that covers the mounting table. The light guiding member and the LED array on the mounting table are fixed in place by the cover.
Although the cover and the mounting table hold the light guiding member without forming screw holes on the light guiding member, this approach also has a drawback in that the cover needs to be somehow fixed to the light guiding member or the mounting table, resulting in an increase in height. In addition, although the light guiding member and the LED board on the mounting table are fixed by the cover, it is not clear how the light guiding member and the LED board are positioned and fixed in place.